iommu/amd: Clear DMA ops when switching domain

In the Linux kernel, the following vulnerability has been resolved:

iommu/amd: Clear DMA ops when switching domain

Since commit 08a27c1c3ecf ("iommu: Add support to change default domain

of an iommu group") a user can switch a device between IOMMU and direct

DMA through sysfs. This doesn't work for AMD IOMMU at the moment because

dev->dma_ops is not cleared when switching from a DMA to an identity

IOMMU domain. The DMA layer thus attempts to use the dma-iommu ops on an

identity domain, causing an oops:

# echo 0000:00:05.0 > /sys/sys/bus/pci/drivers/e1000e/unbind

# echo identity > /sys/bus/pci/devices/0000:00:05.0/iommu_group/type

# echo 0000:00:05.0 > /sys/sys/bus/pci/drivers/e1000e/bind

...

BUG: kernel NULL pointer dereference, address: 0000000000000028

...

Call Trace:

iommu_dma_alloc

e1000e_setup_tx_resources

e1000e_open

Since iommu_change_dev_def_domain() calls probe_finalize() again, clear

the dma_ops there like Vt-d does.